National Aeronautics and Space Administration UNIVERSE DISCOVERY GUIDES September MILKY WAY GALAXY: CITY OF STARS This view of our Milky Way Galaxy is a panoramic view of the entire sky as seen by the Two Micron All-Sky Survey. The image is centered on the core of our Galaxy, toward the constellation of Sagittarius. IN THIS GUIDE » MILKY WAY GALAXY: CITY OF STARS » SKY FEATURE: MILKY WAY GALAXY » TRY THIS! » ACTIVITY: THE MILKY WAY PROJECT » CONNECT TO NASA SCIENCE » Acknowledgements » Appendix: September Star Map Published 2013. The universe is a place of change. NASA missions advance our understanding of the changing universe. www.nasa.gov 2 MILKY WAY GALAXY: CITY OF STARS Imagine flying in an airplane high over a large city at night. Looking down you see a lot of lights. The bright dense city center is surrounded by arms of light punctuated with smaller glowing knots. Farther out are open areas with few or no lights at all. From your distant perspective, the whole scene looks quiet, peaceful and unchanging. Minneapolis and surrounding area at night. Credit: NASA Earth Observatory/NOAA NGDC But if you could take out a large telescope and take a closer look, you’d find a bustling city that holds an abundance of activity, change and renewal. Traffic buzzes about. Office buildings, shops and streets are filled with people conducting business. Hospitals shelter babies that are being born. New buildings are being constructed on the sites of recently demolished buildings. Looking at the surrounding knots of light you find smaller communities. They may be less densely populated, but here too you find activity and change. Homes and shopping malls are being built, new people move into communities and some people move out. Children grow up and older people pass on. The open areas with few lights hold ranches, farms, forests, meadows, lakes and mountains. Out here, far from the city lights, people can look up and enjoy the beautiful dark skies above — where your airplane is flying overhead. 3 Now land back on Earth. Stand outside and look out at the stars of our Galaxy. The sky looks quiet, peaceful and unchanging. But if we look closer, we get a very different impression. NASA telescopes allow us to take that closer look into our Galaxy. We’ve found a bustling metropolis full of activity, change and renewal. The densely-populated center of our Galaxy holds a massive black hole around which stars orbit at a fantastic pace. Throughout the disk of our Galaxy, large clouds of gas and dust mark the birthplaces of new stars. Hot young stars force out the gas and dust surrounding them. Aging stars expand outward and cool off as they use up their nuclear fuel. X-ray radiation blasts from sites of black holes. Radio signals mark the pulses of energy from a spinning neutron star. Artist’s rendering of the Milky Way Galaxy based on Spitzer infrared space telescope data. Credit: NASA/JPL-Caltech/R. Hurt (SSC/Caltech) Out above the disk of our Galaxy, a few rogue stars and tight clusters of aging stars slowly orbit the center of the Galaxy. If you could be out there, you could look down on the disk of the Galaxy and see the bright dense Galaxy center surrounded by arms of stars punctuated with smaller glowing knots of newborn stars. The peaceful-looking scene belies the bustling metropolis within. 4 SKY FEATURE: MILKY WAY GALAXY How to Find it Distance to Center of Galaxy: 26,000 light-years To view the band of the Milky Way: A dark sky and just your eyes Click here to jump to the full-sky September Star Map. Look up at night — all the stars you see are in our own Milky Way Galaxy. The Sun and its planets, including Earth, are embedded in the Milky Way Galaxy. Our Galaxy is a disk of stars, gas, and dust, shaped somewhat like an enormous DVD — wide, circular, and thin. We are embedded in the disk looking out through it. On September evenings, if you can find a dark place away from bright city lights, you can see the hazy band of starlight we call the Milky Way runs from the southwestern horizon across the sky to the northeastern horizon. When we view the hazy band of starlight, we are looking from within the disk of our Galaxy, out through the thickest part of that disk. The center of our Galaxy is in the direction of the constellation of Sagittarius. In September, you can find that constellation above the southern horizon. Use binoculars to look in that direction and you will see a multitude of stars appearing to be crowded together because the stars are more densely packed in the center. Just a hundred years ago, almost everyone could step outside on a clear evening and see the band of the Milky Way arching overhead. Today with our large brightly-lit cities, it is washed out due to all the streetlights. Starlight traveling Center of the thousands of years across the Milky Way Galaxy is lost to our eyes in the last fraction of a second of its SAGITTARIUS journey to Earth. Only by going far out in the countryside will you see that band of light. That is one of the reasons we put telescopes in remote locations, on mountaintops, and out in space. 5 TRY THIS! How do I imagine the billions of stars in our Galaxy? Imagine if you could shrink our Solar System, the Sun and all its planets, so it fits into the palm of your hand. On this scale the Sun would be smaller than a tiny grain of sand. With the Solar System sitting in the palm of your hand, the Milky Way Galaxy, with its 200–400 billion stars, would span North America — about 2,500 miles (4,000 km) across. Imagine a football field with a four-foot (1.3 meter) high wall built all Credit: NASA/ASP around the edges. We’ll use birdseed to represent the stars of our Galaxy. Fill the football field to the top of that wall with birdseed. That’s roughly 200 billion birdseed representing 200 billion stars of our Galaxy. For more on this model, watch the video on this page: http://nightsky.jpl.nasa.gov/download-view.cfm?Doc_ID=334 Curious about Black Holes in our Galaxy? To learn The Truth about Black Holes, visit this interactive site: http://amazing-space.stsci.edu/resources/explorations/blackholes/lesson/ http://amazing-space.stsci.edu For print-friendly information on black holes, visit: http://teachspacescience.org/graphics/pdf/10001043.pdf For more Hubble education and public outreach activities from the Space Telescope Science Institute, visit: http://amazing-space.stsci.edu/ For more resources related to black holes from the Fermi Mission’s education team at Sonoma State University, visit: http://glast.sonoma.edu/teachers/blackholes/ 6 ACTIVITY: THE MILKY WAY PROJECT Time: A few minutes to as much time as you want to spend Age: 10 and up Help scientists identify features of the Milky Way The Milky Way Project (www.milkywayproject.org) has the objective to sort and measure our Galaxy. You can help by looking through images of our Galaxy from the Spitzer and Herschel telescopes. By telling the project scientists what you see in the infrared data, those scientists can better understand how stars form. Try the tutorial now: http://www.milkywayproject.org/tutorial/bubbles For more citizen science projects, visit the Zooniverse website: https://www.zooniverse.org/ Looking for more Earth and Space Science formal and informal education activities? Try out NASA’s digital collection of resources at NASA Wavelength: http://nasawavelength.org http://nasawavelength.org 7 CONNECT TO NASA SCIENCE How do we know? How do scientists know about all the activity and change in the Milky Way? When we look with just our eyes, most of our Milky Way Galaxy’s http://science.nasa.gov stars are hidden behind thick clouds of galactic dust and gas. Milky Way as our eyes see it. Credit: Axel Mellinger From Earth’s surface, we cannot detect much of the infrared and high-energy radiation from deep within our Galaxy. By using space telescopes, like Spitzer, Herschel and Swift, that detect this radiation, we can see through the dust and gas clouds, and discover the secrets hidden from our eyes and Earth-bound telescopes. http://www.spitzer.caltech.edu/images/2680-ssc2008-11a-Spitzer-Finds- Clarity-in-the-Inner-Milky-Way More than 800,000 frames from NASA’s Spitzer Space Telescope were stitched together to create this infrared portrait of dust and stars radiating in the inner Milky Way. Credit: NASA/JPL-Caltech/Univ. of Wisconsin. For the latest news from Spitzer, visit http://www.spitzer.caltech.edu/news 8 NASA’s Swift Gamma-Ray satellite discovers a new black hole in our Galaxy. Watch the video here: http://www.nasa.gov/mission_pages/swift/bursts/new-black-hole.html For more news from Swift: http://swift.gsfc.nasa.gov/docs/swift/news/ Stellar Nursery in our Galaxy’s Core NASA’s SOFIA infrared telescope spots a ring of gas and dust around the Galaxy’s central black hole and a neighboring cluster of infant stars. http://www.sofia.usra.edu/News/news_2013/01_08_13/index.html For more news from SOFIA: http://www.sofia.usra.edu/News/news_updates.html 9 NASA’s NuSTAR X-Ray telescope catches the Galaxy’s central black hole nibbling on a snack that ventured too close. http://www.nasa.gov/mission_pages/nustar/news/nustar20121023.html First Look at Milky Way’s Monster in High-Energy X-ray Light.